Actually you don't need to print it ar all unless it's a reservation for a night train, where it might be good with a printed copy.

You can show mandatory reservation on your phone and optional reservation you normally don't show at all, unless there is a person sitting on your seat who refuses to move.

Thank you soo much

But the phone reservation should not be shown in the email I guess 

I’m coming to europe for the first time and I don’t want to mess up

You show the reservation in the way you can show it, often it is a pdf in an email that you show on your phone. So make sure that you can access your email on your phone.